There has been a lot of discussion as to whether builders sand contains salt, if it did the concrete would not set correctly so 'yes' you can dig into your vegetable beds but don't over do it and add other organic matter too, compost, well rotted animal manure, spent mushroom compost.

I would say that if it is sharp sand (coarse, which is used for concrete) then it will break up the clay soil and I would also use it spread thinly on a lawn. If it is soft sand (fine sand that is used in mortar for brick laying), then I wouldn't use it as it will clog the soil particles.

Some builder's sand is washed, some isn't, hence the confusion about salt levels - washed can be used on planting areas, but its not a good idea to use unwashed.